Synergistic CH4 hydrate recovery and CO2 storage by coupling depressurization with CO2/N2 injection: A pilot-scale investigation

To ensure sustainable energy supply while mitigating climate change, combining CH4 recovery with CO2 storage in marine natural gas hydrate (NGH) reservoirs is a promising carbon-neutral technology. In this study, a pilot scale reactor (V = 22 L) was used to study the combined processes of CH4 recovery from the methane hydrate bearing sediment by using depressurization followed by in-situ CO2/N2 injection to form mixed CH4/CO2/N2 hydrate (Mix-H) at marine NGH reservoir conditions (T = 283.8 K and P = 11.0 MPa). The bottom hole pressure (BHP = 3.2 MPa, 5.3 MPa, and 6.8 MPa) during depressurization and the setting of horizontal wellbore (HW), were systematically investigated to optimize the synergistic CH4 recovery and CO2 storage. The CH4 recovery ratio (55.4-80.6%) and the CH4 production rate (5.15-134.98 mol/d) increased as BHP decreased. After CO2/N2 injection, Mix-H formed first in the CH4-rich region in the upper section of the reactor and propagated downward to the lower. The Mix-H formation kinetics were significantly enhanced with an increase in BHP, leading to a decrease in CO2 composition in the Mix-H. The use of a horizontal wellbore (HW) facilitates a more even distribution of the CO2/N2 within sediments, resulting in a 13.5% increase in CO2 composition of Mix-H compared with no HW setting. The CO2 conversion ratio and hydrate restoration ratio reaches 52.8% and 69.4% at BHP = 5.3 MPa with HW, respectively. The technical feasibility of CO2 storage in exploited marine NGH reservoirs was first proved at pilot-scale. The results provided insights into the synergistic method of NGH exploitation and CO2 storage at the marine NGH reservoirs.